Low-molecular-weight gelators (LMWGs) are capable of assembling into interwoven fibrillar networks that entrap solvents between strands to form thermoreversible supramolecular gels.[1-10] Chirality has a profound influence on the macroscopic gelation of solvents by facilitating the growth and stabilization of noncovalent helical fibers as well as their interwoven networks, often driven by stereogenic centers present in the molecular structures of chiral LMWGs.[11-13] As a consequence, most of the highly efficient LMWGs, exhibiting strong gelling ability, are composed of enantiomerically pure chiral molecules.[1, 14-16] Generally speaking, the corresponding racemates of these enantiopure chiral gelators, either do not form gels or occasionally form only weak ones that transform readily into precipitates or discrete crystals.[12, 17, 18] The opposite situation, in which a racemate generates a gel, while both its enantiomers are less efficient gelators, or even lack any gelling ability at all, is rare. Although there are a few examples of gels resulting from the assembly of racemic gelators incorporating flexible structures, driven by means of various noncovalent bonding interactions, gels assembled from highly rigid racemic gelators at the behest of multiple weak [C—H . . . O] interactions as the major driving force remain unexplored to the best of our knowledge.[18-26, 27] Kim et al. have reported that rigid achiral cucurbit[7]uril (CB[7]) can act as a hydrogelator, but only in the presence of mineral acids.[28] The relationship between stereochemistry and gelation, however, has yet to be fully elucidated.
Hydrogen-bonding arrays are well-established modules for the formation of biotic and abiotic supramolecular polymers, as well as for the assembly of cylindrical and spherical capsules.[29, 30, 31-36] While a number of planar quadruple hydrogen-bonding motifs give rise to supramolecular arrays, cyclic peptides are amongst the few well-known examples of multiple hydrogen-bonding ring motifs that lead to the formation of supramolecular nanotubes.[37-43] Also, despite the remarkable progress that has been made in recent years, interactions involving hydrogen-bonding motifs have been restricted for the most part to the use of [O—H . . . O] and [N—H . . . O] noncovalent bonds because of their greater strengths and propensities to act cooperatively. [29, 30, 41, 44-49] These strong noncovalent bonds facilitate the construction of well-defined supramolecular assemblies by over-riding the influence of other competing interactions from mismatched molecular structures, counter ions and solvents. Permutations of hydrogen bonds composed of multiple intermolecular cooperative [C—H . . . O] interactions, leading to the formation of supramolecular assemblies, have remained largely out of reach on account of the relative weakness of single [C—H . . . O] interactions.[49]
There are a number of applications for the supramolecular assemblies. For example, supramolecular assemblies may be used to prepare batteries, organic semiconductors, including but not limited to organic field effect transistors, organic light emitting diodes, and photovoltaic devices, membranes, fibrous networks, or gas sensors. As a result, there is a need for new supramolecular assemblies.